1. Field of the Invention
This invention relates to an electrochromic display device for displaying information on a transparent plate, and more particularly to an electrochromic display device composed of a pair of oppositely disposed electrode plates and an electrochromic composition coloring in a reduced state sealed between the pair of electrode plates. 2. Description of the Prior Art
It is known that electrochromic materials present different colors depending upon the state of the materials, i.e. oxidized state or reduced state, since the spectroscopic absorption of the materials differs depending upon the state thereof. As the electrochromic materials are known inorganic electromic compounds such as tugsten oxide and molybdenum oxide, and organic electrochromic compounds such as pyridin compounds and aminoquinon compounds. The electromic compounds are divided into electrochromic compounds coloring in a reduced state and those coloring in an oxidized state. The electrochromic compounds coloring in a reduced state does not show a color or shows a light color in an oxidized state and colors in a reduced state. The electrochromic compounds coloring in an oxidized state does not show a color in a reduced state and colors in an oxidized state.
The electrochromic compounds are therefore sealed between a pair of electrodes at least one of which is transparent and subjected to an electric field applied thereacross. When the electric field application is turned on and off across the space between the pair of electrodes, the color of the electrochromic compound sealed therein changes. Therefore, by making the electrodes into a patterned form, a pattern display can be effected. Actually, patterned transparent electrodes are applied on the inner surface of a pair of electrode support plates.
In the conventional electrochromic display device, the electrochromic compound is sealed in the space between the pair of electrodes in an oxidized state. When the pair of electrodes are supplied with a voltage, the electrochromic compound in the vicinity of the cathode side electrode is colored. Therefore, the display is conducted in the form of the pattern of the cathode side electrode.
The above described electrochromic display device (hereinbelow referred to simply as "EC display") has a defect as follows. In a sealed system wherein the electrochromic compound (hereinbelow referred to simply as "EC compound") is isolated from the ambient atmosphere, a part of the EC compound becomes non-reversible by the influence of a hydrogen gas or the like generated by the electrolytic effect of water added to the EC compound as a solvent. Therefore, a part of the EC composition sealed in the EC display remains to be reduced even after the application of electric field is turned off. Further, since the EC compound has a little solubility in the reduced state, the colored EC compound in the reduced state gradually diffuses on the display face of the EC display, which results in degradation in the quality of image displayed.
Since the EC compound conducts the display by the reversible change in color between the oxidized and reduced states, the display once made will not disappear until the reduced EC compound diffuses through the solvent as water in the EC compound. Therefore, the conventional EC display is normally of so-called memory type which maintains the displayed pattern for a long time. In practice, there is known a memory type EC display which is capable of maintaining its displayed pattern for several months.
The memory type EC display as described above is suitable for a display device which is designed to maintain the displayed pattern for a long time. However, this type of EC display is not proper to a display device in which the displayed pattern is required to frequently change as in the dial of clocks or watches. In those cases where the displayed pattern should be frequently changed, the displayed pattern by application of an electric field in one direction is erased by application of an electric field in the opposite direction as shown in U.S. Pat. No. 3,774,988.
The above described memory type EC display which needs an electric circuit for reversing the direction of application of the electric field is disadvantageous in that the circuit structure gets complicated.
On the other hand, it has been known in the art to show and erase a displayed pattern only by turning on and off the power supply to the EC display as in U.S. Pat. No. 3,774,988. In this EC display device, an EC compound of oxidation state is sealed in a cell with a small opening. The EC compound is reduced when it is subjected to an electric field and the reduced compound is returned to its oxidation state by contact with air by way of said opening when the electric field is turned off.
The above described EC display in which the reduced EC compound is returned to its oxidation state to erase the displayed pattern only by turning off the electric field is disadvantageous in that the area of the displayed pattern cannot be made so large since the oxidation is conducted through the contact of the EC compound with the oxygen in air by way of the small opening provided in the display device. Further, since water is used as the solvent, the water will be vaporized through the opening and accordingly the life of the display is not so long.